A. Principles of Operation
Secondary cells using rechargeable hydrogen storage negative electrodes are an environmentally non-threatening, high energy density electrochemical power source. These cells operate in a different manner than lead acid, nickel-cadmium or other battery systems. The rechargeable hydrogen storage electrochemical cell or battery utilizes a negative electrode that is capable of reversibly electrochemically storing hydrogen. These cells usually employ a positive electrode of nickel hydroxide material, although other positive materials may be used. The negative and positive electrodes are spaced apart in an alkaline electrolyte, which may include a suitable separator, i.e., a membrane, therebetween.
Upon application of an electrical potential across the cell, the negative electrode material (M) is charged by the electrochemical absorption of hydrogen and the electrochemical evolution of hydroxyl ion: EQU M+H.sub.2 O+e.sup.- M--H+OH.sup.- (Charging)
Upon discharge, the stored hydrogen is released to form a water molecule and evolve an electron: EQU M-H+OH.sup.- M+H.sub.2 O+e.sup.- (Discharging)
In the reversible (secondary) cells of the invention, the reactions are reversible.
The reactions that take place at the positive electrode of a secondary cell are also reversible. For example, the reactions at a conventional nickel hydroxide positive electrode as utilized in a hydrogen rechargeable secondary cell are: EQU Ni(OH).sub.2 +OH.sup.- NiOOH+H.sub.2 O+e.sup.- (Charging), EQU NiOOH+H.sub.2 O+e.sup.- Ni(OH).sub.2 +OH.sup.- (Discharging).
A secondary cell utilizing an electrochemically rechargeable hydrogen storage negative electrode offers important advantages over conventional secondary cells and batteries, e.g., nickel-cadmium cells, lead-acid cells, and lithium cells. First, the hydrogen storage secondary cells contain neither cadmium nor lead nor lithium; as such they do not present a consumer safety or environmental hazard. Secondary, electrochemical cells with hydrogen storage negative electrodes offer significantly higher specific charge capacities than do cells with lead or cadmium negative electrodes. As a result, a higher energy density is possible with hydrogen storage cells than with conventional systems, making hydrogen storage cells particularly suitable for many commercial applications.